IMPORTANT NOTE: The Dual Maps tool only works when using a Map Template that contains a service with the projection Web Mercator Auxiliary Sphere. This limitation is a requirement of the various various target map systems (Bing, GoogleMaps, StreetView,..)

If you add the custom tool to a Map that uses a template containing a single map service (ex :GDA 94), the result will open but the contents of the tool remains blank.
The solution is to change the projection of the Map Service used in the template to Web Mercator Auxiliary Sphere.